1. Copy race mission folder and add "_mirror" suffix to its name,
2. Run Check.bin editor, open check.bin from newly created folder and locate starting/finish line,
3. Switch IDs as shown on the screenshoot (IDs 2 & 3 are track bounds, 636 - start/finish line). If there are more points between boundaries switch them as well,

4. Run MWE and rotate starting points. Make any other changes you find necessary.

I advise to download Polish version of Check.bin editor so you can at least translate stuff: Download Link

There's a quirk in Check editor. You can't switch IDs in one go. First you have to "free" these particular IDs. So set them to 0, save file, reopen it again (I don't remember if restarting program is also needed), and only now you are able to set them to IDs you want.